1. Field of the Invention
The present invention relates to a light deflecting apparatus in which a sleeve of ceramic material is rotatably fitted onto an axis or a shaft of ceramic material, a rotary polygonal mirror is mounted to a rotor portion comprised of the axis or the sleeve, and the rotor portion is rotated by a driving motor. Such a light deflecting apparatus is used in an image forming apparatus, such as a laser beam printer and a laser facsimile apparatus.
2. Related Background Art
Conventionally, a bearing apparatus rotatable at fast speed and with high precision is required to increase precision of such a light deflecting apparatus. A dynamic pressure fluid bearing of ceramic material rotatable in a non-contact manner is used for such a bearing apparatus.
FIG. 1 illustrates a light deflecting apparatus with a dynamic fluid bearing used as a bearing apparatus which is disclosed in Japanese Patent Application Laid-Open No. 8-5951.
A stationary axis or shaft 22 of ceramic material is fixed to a housing 21 of a driving motor. A rotary sleeve 24 of ceramic with a cover 23 at its upper end is rotatably fitted onto the stationary axis 22. A flange 25 of metal, such as aluminum and brass, is fixed to the periphery of the rotary sleeve 24 using shrinkage fit or the like. A driving magnet 26 is fixed to the periphery of the flange 25 using adhesive or the like. Further, a base plate 27 is fixed onto the housing 21, and a stator 28 is arranged on the base plate 27 opposingly to the driving magnet 26. The driving motor is thus constructed.
On the other hand, a first permanent magnet 29 is mounted to a lower end of the rotary sleeve 24. A second permanent magnet 30 is fixed onto the housing 21 such that different magnetic poles of those magnets 29 and 30 are opposed to each other in a vertical direction. Thus, the rotary sleeve 24 is floated by such magnetic reaction, and an air pool 31 is formed between the stationary axis 22 and the cover 23. A through hole 23a is formed in the cover 23 to make a vent between the air pool 31 and its exterior. A plug 32 is removably fitted into the through hole 23a. Further, a rotary polygonal mirror 34 is fixed to the flange 25 by a leaf spring 33 fixed to the rotary sleeve 24.
With such a structure, when the rotary sleeve 24 is to be fitted onto the stationary axis 22 with the plug 32 detached from the hole 23a, the sleeve 24 can be readily fitted onto the axis 22 since air is vented from the sleeve 24 through the hole 23a. Further, when the rotary sleeve 24 is rotated, the sleeve 24 is supported in a radial direction by an air film between the sleeve 24 and the axis 22 and supported in a thrust direction by the reaction acting between the permanent magnets 29 and 30. At this time, the air sealed within the air pool 31 by the plug 32 functions to damp a vertical motion of the rotary sleeve 24, so that the rotary sleeve 24 is stably maintained at its floated position.
The above-discussed bearing apparatus comprised of those stationary axis and the rotary sleeve, however, has the following disadvantages.
When the flange 25 of metal is fixed to the rotary sleeve of ceramic using the shrinkage fit, a bore of the sleeve 24 is likely to deform and hence after-working is required, leading to an increase in cost.
Further, when the polygonal mirror 34 is to be fitted onto the rotary sleeve 24 of ceramic, precise working of the periphery of the sleeve 24 is needed, also leading to an increase in cost.
Furthermore, when a groove is formed on the rotary sleeve 24 of ceramic to fit therein the leaf spring 33 for fixing the polygonal mirror 34 onto the flange 25, ceramic used for the sleeve 24 is likely to be cracked or broken and there is a fear that cracked powders get in the dynamic pressure bearing to damage the bearing. Moreover, formation of the groove on the periphery of the sleeve of ceramic is difficult, leading to an increase in cost.